X-ray tube anodes emit just a fraction of the energy beamed into them in the form of X-ray radiation. The remainder is converted to heat and must exit the anode in the form of heat radiation.
For many years, the state of the art has been familiar with methods conceived to improve the thermal emissivity of X-ray tube anodes made of refractory metals by employing an oxide coating on the surface of the anodes (AT 337 314, DE-OS 22 01 979, DE-OS 24 24 43 354). These publications disclose various oxide materials and fabrication techniques, and lay claim to the ability to increase the adhesion of the oxide layer on the surface of the host metal vis-a-vis the state of the art and to raise the thermal emissivity of the anode surface.
It has been shown that the capacity of layers manufactured in accordance with such methods and techniques has not been able to keep pace with the increasing requirements for such products, in view of the ever increasing demands placed on X-ray tube anodes with respect to layer ageing, thermal reflectivity and resistance to degasification (prevention of electrical flashovers).
EU A2 0 172 491 discloses, in a further development, an X-ray tube anode, made of a molybdenum alloy, having an oxide coating consisting of a mixture of 40-70% titanium oxide, with the remainder of the coating comprising stabilizing oxides from the ZrO.sub.2, HfO, MgO, CeO.sub.2, La.sub.2 O.sub.3, and SrO group. In order to better satisfy the previously mentioned demands placed on such layers, EU A2 172 491 proposes fusing the oxides so as to form smooth, glossy, gleaming layers.
EU A2 0 244 776 essentially pertains to the same subject matter. The publication relates to the preprocessing of the oxide material, prior to its application to the X-ray tube anode, by means of standard spraying techniques. Accordingly, in an initial processing step, a mixture consisting of 77-85% in weight of titanium oxide, with 15-23% in weight of calcium oxide, is processed to a powder mixture having a homogeneous phase. Thereafter, this mixture is applied to the X-ray anode (and, if necessary, in mixture with other oxide powders) in accordance with spraying methods known in the art. Plasma spraying, sputtering methods, chemical and physical precipitation processes from the gas phase, and electron beam methods are named as layering processes to be used in the application of an oxide coating to X-ray tube anodes made of refractory metals. Additionally, for X-ray tube anodes made of refractory metals, it is usual that the anodes undergo degasification annealing at the conclusion of the manufacturing process. The degasification annealing serves to prevent gas leakage from the anode, along with the resulting, highly undesirable, plasma flashovers between the electrodes when the anodes are used in an X-ray tube in a high vacuum.
The prior publication thus discloses a formulation of the oxide layer, with respect to annealing processing, following coating of the X-ray tube anodes. Degasification annealing simultaneously promotes final formation and fusing of the oxide phase, which is unachievable by an oxide application process alone. However, in view of the ever increasing demands placed on X-ray anodes, the composition and manufacturing processes for oxide layers disclosed in EU A2 244 776 are deficient. In fact, the annealing process disclosed in this prior printed publication presents the danger of an unacceptable degree of interfusion of the oxide layer, in the area of the focal path, at the border between the coated and uncoated portions of the surface of the X-ray tube anode. This occurs because the annealing temperature required to fuse the oxides into smooth, satisfactorily adherent layers renders the layers highly fluid.
In addition, such oxide layerings exhibit an unwelcome gas phase formation at the requisite annealing temperatures.